1. Field of the Invention
The present invention relates to an image processing apparatus, an image processing method, and a program. More particularly, the invention relates to an image processing apparatus, an image processing method, and a program such that when an image signal before interpolation is cleared, the image quality of the image signal following interpolation is improved.
2. Description of the Related Art
FIG. 1 schematically shows a typical structure of an ordinary image processing apparatus 1.
In FIG. 1, the image processing apparatus 1 is made up of an image clearing section 11, an OSD multiplexing section 12, a frame rate converting section 13, a panel driving section 14, a display panel 15, and a control section 16. Illustratively, the image processing apparatus 1 displays chronologically ordered frame-by-frame images of a TV program by raising the frame rate of image signals that are the digital signal of each pixel.
In response to an image clear command signal supplied by the control section 16, the image clearing section 11 clears the image signal of an externally input TV program. More specifically, the image clearing section 11 turns the level of the image signals for all pixels to black (i.e., zero). The image clearing section 11 supplies the OSD multiplexing section 12 with the image signal having undergone the image clearing operation or with the input image signal without modification.
In accordance with a multiplex command signal coming from the control section 16, the OSD multiplexing section 12 multiplexes the image signal supplied by the image clearing section 11 with an image signal representative of OSD (on-screen display) images such as channel numbers and menus. The OSD multiplexing section 12 supplies the frame rate converting section 13 with the multiplexed image signal or with an unmodified image signal as supplied from the image clearing section 11.
Given the image signal from the OSD multiplexing section 12, the frame rate converting section 13 carries out a frame rate enhancement process for raising the frame rate (i.e. cycle) of the received image signal. More specifically, the frame rate converting section 13 first performs a frame interpolation process on the image signal supplied from the OSD multiplexing section 12. The image signal resulting from the interpolation is output to the panel driving section 14 as an image signal that is interposed chronologically between the externally input image signals. The process makes the frame rate of the input signal to the panel driving section 14 higher than the frame rate of the signal entering the image clearing section 11.
The panel driving section 14 performs D/A (digital/analog) conversion and other processes on the image signal supplied from the frame rate converting section 13. The resulting analog signal is used by the panel driving section 14 to drive the display panel 15 such as a liquid crystal display (LCD) panel whereby frame-by-frame images are displayed.
The control section 16 receives from a user such commands as an input change command for changing input sources of image signals, a channel change command for changing TV channels conveying image signals, and an OSD image display command. In response to the input change command or channel change command from the user, the control section 16 feeds an image clear command signal to the image clearing section 11. The operation makes it possible to prevent the image display from getting distorted upon input change or channel change. The control section 16 further supplies the OSD multiplexing section 12 with the multiplex command signal in accordance with OSD image display instructions.
The frame rate enhancement process performed by the frame rate converting section 13 in FIG. 1 is indispensable for improving the image quality of chronologically ordered image signals. The same holds true for frame interpolation used in the process for compensating the motions of such chronologically ordered image signals.
One way to implement the frame interpolation process involves first detecting a motion vector from chronologically input image signals and then using the detected motion vector to interpolate image signals at certain points in time between chronologically ordered image signals (as disclosed in Japanese Patent Laid-Open No. 2001-42831).
The motion vector is detected illustratively using the so-called block matching method whereby chronologically ordered image signals are compared with one another. In this case, the frame rate converting section 13 compares an image signal that has been cleared of its image with an image signal yet to be cleared at the beginning or at the end of an image clearing operation. As a result, an inaccurate motion vector is detected and used to generate an incorrect image signal, which leads to deterioration of the image quality of image signals subsequent to interpolation.
Illustratively, suppose that with an image clearing operation started as shown in FIG. 2A, the image signal of an uncleared image 21 having a black donut-shaped circle against a white background is fed to the frame rate converting section 13 followed by the image signal of a cleared image 22. In such a case, as shown in FIG. 2B, the ideal is that the frame rate converting section 13 interpolates the image signal of an image 23 identical to the image 21 and that the identical image 23 is kept displayed immediately up until the cleared image 22 is displayed.
In practice, the frame rate converting section 13 detects the motion vector by comparing the image signal of the uncleared image 21 with the image signal of the cleared image 22. The motion vector thus detected is used to interpolate the image signal of an aberrant image 24 having a black-and-white striped donut-shaped circle against the black background as shown in FIG. 2C. The result is a deteriorated quality of the image immediately before the cleared image 22.
Although not shown, the image signal of the cleared image is compared likewise with the image signal of the subsequent uncleared image at the end of the image clearing operation. The result is a similarly degraded quality of the image immediately before the uncleared image.